In those patents, a web of flexible material such as a fabric, paper or metal, passes partly around a roller rotatably supported on an axle, the ends of which axle are in turn supported by a device capable of measuring the transverse or radial forces exerted on the roller by the tensions in the web. In each of these patents, the device consists generally of a single, metal, flexible beam or arm integral with and cantilevered at one end on a base plate mounted on a side frame of the apparatus and at the other end supporting a face plate which in turn is operatively associated with an end of the roller axle through a hinged or jointed connection which enables the extended line of the end of the beam or arm to diverge slightly from the axis of the roller as the beam or arm bends under the forces imposed thereon by the axle. In each of these patents, one or more strain gages of conventional construction are permanently fastened to the sides of the beam or arm generally close to the point where the beam or arm is cantilevered from its base plate. Such flexure of the beam or arm under forces imposed by the moving web stresses the gages and alters their internal resistance. Apparatus is then provided for measuring the changes in the resistance of the strain gages due to the stress variations therein, and either visually indicating the changes or actuating other servo-mechanisms for maintaining the web tension constant. Such apparatus normally includes electronic amplifying means of varying degrees of amplification.
Such devices are quite sensitive to small force variations, so much so, in fact, that the response curve has an undesirable hysteresis loop therein. Analysis showed that it was caused by the friction in the hinged or jointed connection between the axle and the movable face plate. Thus, as tension on the web increases, the beam bends, resulting in its extended axis diverging relative to the roller axis, which divergence is taken up by the hinged connection. Undesirable friction in this connection produces a force couple which, as web tension increases, acts to resist the bending of the beam and therefore causes a reduction in the stress applied to the strain gage and the resultant tension signal. As tension in the web decreases and the beam is unloaded, the friction couple in the hinged connection produces a force couple which acts in the opposite direction and causes an increase in the resultant tension signal. The difference in these signals results in a hysteresis loop in the response curve which makes the sensor unsatisfactory for extremely critical applications.
Subsequently, a sensor was introduced comprised of a pair of parallel spaced arms or beams each integrally connected at one end to the base plate and at the other end to the face plate which face plate then was connected to the axle of the web roller.
As this twin beam sensor was loaded, the extended axis from the face plate remained parallel to the unloaded axis and no force couple was introduced due to the bending of the beams. Accordingly, it was found possible to eliminate the expensive bearing connection and a rigid connection between the shaft and the sensor was employed.
Such devices were an improvement, even though, in some instances, a hysteresis loop remained. As a result of observations which lead to the development of the present invention it was found that, particularly when the web roller had a substantial length, the axle of the roller was bent or deflected throughout its length by the web tension forces such that its extended axis was no longer parallel to the extended axis of the face plate. This placed one of the beams in longitudinal tension and the other in longitudinal compression which forces affected the strain gages in the same manner as bending forces and gave inaccurate results and hysteresis in the response curve. Further, the arrangement had difficulty accommodating these misalignments of the shaft and/or expansions or contractions thereof.
A further problem which is common to prior web tension transducers employing a flexible diaphragm for coupling the shaft to the supporting face plate is the likelihood of damage or failure to the flexible diaphragm due either to fatigue caused by repetitive overloading of the transducer or vibration of the roller shafts, or to repeated shaft expansion such as results when the roller shaft is heated. Also, prior web tension transducers which have employed a ball bearing pivot for the hinged or jointed connection between the shaft and the movable face plate have experienced the additional problem of having a very limited ability to accommodate shaft expansion.
A further problem which has come to attention since the filing of the parent application is a fretting type corrosion in an articulated joint such as employed in the preferred embodiment of the invention, which corrosion resulted in inaccuracies of measurement. As is well known, in web tension measuring devices the radial forces on the web rollers are very high. In the web tension sensor according to the invention, these forces are transmitted to the sensor through an articulated joint made of metal. In operation, the angular variation in this joint, particularly those produced by vibration, is substantially less than 1.degree.. Analysis showed that the lubrication film between the metal surfaces broke down under the high unit pressures caused by the radial forces on the roller, resulting in a metal-to-metal contact. The very small angular movement in the joint caused a molecular bonding of the metals forming the joint which resulted in what appears to be a fretting corrosion or galling of the bearing surfaces as the angular movement continued. This fretting corrosion of the bearing surfaces then caused the aforementioned inaccuracies of measurement.